This app doesn’t directly support importing Google Voice messages but there are various scripts developed by users of this app that you can use to convert the Google Voice messages exported using Google Takeout into the XML format used by SMS Backup & Restore.

One of the most complete implementation is at https://github.com/SLAB-8002/gvoice-sms-takeout-xml

Please note that we haven’t used these scripts ourselves so you will have to try it out yourselves and check if it works for your use case.